1. Field of the Invention
The present invention relates to an equalizer of a wireless communication apparatus, more particularly to an equalizer of a wireless communication apparatus for estimating transmission line characteristics by using a transmission line estimation sequence, which is a symbol for estimating transmission line characteristic of each sub-carrier.
2. Description of Related Art
A transmission line estimation error has often occurred conventionally on a condition that combines two functions; one of the functions is an interference avoiding function for avoiding interferences with other wireless communication devices and the other function is a transmission line equalizing function that uses an equalizer for reducing degradation of characteristics to occur due to a fading process. The problem has been specific to wireless communication systems. Hereunder, there will be described such interferences with other wireless communications and the fading process.
In the case of the UWB (Ultra Wide Band) wireless communication that uses the OFDM (Orthogonal Frequency Division Multiplex) method, wide band frequencies of 3.1 GHz to 10.6 GHz are used. Thus the UWB communication often causes a problem of interferences with other wireless communication devices that use those bands. To avoid such a problem, a UWB wireless communication device is provided with an interference avoiding function for detecting frequency bands used by other wireless communication devices and avoiding using those frequency bands and using another frequency band for communications. Concretely, such interferences are avoided with use of a method that belongs to the OFDM communication method characterized in that transmission data is divided into a plurality of carrier waves (hereinafter, to be referred to as “sub-carriers”), then the divided data is sent out.
FIG. 22 shows an explanatory diagram of a spectrum of transmission data while communication interferences are avoided. In the descriptions to be made below, it is premised that a sub-carrier number k and a frequency shown on the horizontal axis are proportional to each other and the 0-th sub-carrier denotes DC components while the vertical axis denotes a power with respect to the subject spectrum. In case where a UWB wireless communication machine detects that other wireless communications use frequency bands corresponding to the sub-carrier numbers 4, 5, and 6 shown in FIG. 22, the UWB wireless communication machine avoids sending sub-carriers of the frequencies corresponding to the sub-carrier numbers 4, 5, and 6 as shown in FIG. 22, that is, avoids assignment of data, thereby avoiding causing of interferences with other wireless communications in the UWB communication and enables both UWB wireless communication machine and other wireless communication machines to be used simultaneously. Hereinafter, avoiding sending of a predetermined frequency sub-carrier will be referred to as “tone nulling”.
As a general problem of wireless communications, there is degradation of communication characteristics to be caused by fading. “Fading” means a phenomenon in which a mutual interference occurs between signals received with different delay times, since the receiver receives signals obtained by synthesizing waves from transmitters with different delay times generated according to various transmission lines. In order to solve this problem, generally, an equalizer provided in the subject receiver estimates frequency characteristics of the transmission line between the transmitter and the receiver (hereinafter, to be referred to as the “transmission line estimation”) and multiplies the received signal by a coefficient obtained through the transmission line estimation (hereinafter, to be referred to as the “transmission line correction coefficient”) to execute the equalization processing. Furthermore, because the signal received by the receiver includes noise, an error occurs in the transmission line estimation.
In order to reduce the transmission line estimation error caused by this noise, the equalizer uses a filter for smoothing transmission line characteristics estimation values. Hereinafter, such a filter for smoothing transmission line characteristics estimation values will be referred to as a “frequency direction filter”. Patent document 1 discloses a technique for reducing degradation of accuracy for interpolating transmission line characteristics obtained with a pilot signal in the frequency axial direction. On the other hand, the non-patent document 1 discloses a frequency direction filter for reducing the transmission line estimation error. Non-patent document 2 describes details of frame configurations of signals to send and receive.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2006-1010119    [Non-patent Document 1] “IDG Information Communication Series 802.11 High Speed Wireless LAN Textbook” supervised by Hideaki Matsue and Masahiro Morikura, IDG Japan Co., Ltd., pp. 192-193    [Non-patent Document 2] “Standard ECMA-368 1st Edition—December 2005, High Rate Ultra Wideband PHY and MAC Standard”, [online], [searched on Oct. 26, 2006], Internet http://www.ecma-international.org/publications/files/ECMA-ST/ECMA-368.pdf, pp. 19-23
However, if smoothing is executed for a tone-nulled sub-carrier in the frequency direction filter, the transmission line estimation error between a tone nulled sub-carrier and its adjacent sub-carrier increases, thereby resulting in degradation of the PER (Packet Error Rate, a possibility of wrong data modulation to occur in a receiver). As a result, a required CNR (Carrier to Noise Ratio), for example, a CNR value at which the PER becomes 8% or under increases. Here, the “CNR” denotes a ratio between a carrier wave and a noise power. This denotes that the larger the CNR value becomes, the less the noise becomes. An increase of the required CNR and an increase of the carrier wave power required for noise are proportional to each other. Consequently, in order to increase the power of a carrier received by a receiver, it is required to shorten the distance between the transmitter and the receiver. If the transmission line estimation error increases in such a way, the required CNR also increases, thereby the communication distance is shortened. This has been a problem.